This invention relates generally to vertical combdrive actuators and to a method of fabrication, and more particularly to a method of fabrication, which provides narrow gap high force combdrive actuators. This invention also describes a double-sided torsional vertical combdrive actuator for increased torsional deflection and a related fabrication method. Finally, this invention presents a fabrication method variation for design of dual-mode vertical combdrive actuators.
There is great demand for high-speed, high-resolution micromirrors in a variety of optical applications including optical scanning, optical switching, free-space optical communications, optical phased arrays, optical filters, external cavity lasers, adaptive optics and other applications. For many of these applications, electrostatic combdrives are the preferred actuation mechanism. Combdrives provide high speed and relatively high force. Furthermore, they can be fabricated using standard materials.
Combdrives produce large deflections at relatively low voltages with continuous stable control over the full range of motion. In vertical combdrives, two sets of comb teeth are staggered in the vertical direction. A voltage applied between the movable top comb array and the static bottom comb array produces a vertical electrostatic force that can be applied to create torsional or piston-like motion. In standard horizontal combdrives the two sets of comb teeth are in-plane (in the horizontal direction) and the piston-like motion is also in-plane in the horizontal direction.
A critical aspect of combdrive design is the spacing or gap between adjacent comb teeth, because the generated force is inversely proportional to this gap. Combdrives with small spacing are, however, more susceptible to misalignment between the interleaved comb teeth. For proper actuator operation, the misalignment tolerance level between the moveable and stationary teeth should be an order of magnitude smaller than the gap width. Micromachined vertical combdrives have been demonstrated in the past. However, the fabrication processes for these devices are either quite complex or create comb teeth that are not accurately aligned.
It is a general object of the present invention to provide a method of fabrication for vertical combdrives with small gaps or spacings between adjacent comb teeth, thereby providing high speed and high force actuation.
It is a further object of the present invention to provide a fabrication process that provides self-aligned vertical combdrives.
It is another object of this invention to provide a double-sided torsional vertical combdrive actuator that increases the range of torsional motion, and a method of fabrication for this device.
Another object of this invention is to present a modified fabrication process and a simple assembly technique to create vertical combdrives for independently controlled torsional and piston motion (dual-mode actuation).
The foregoing and other objects of the invention are achieved by a method of fabrication in which both the top and bottom comb teeth of the vertical combdrive actuator are defined by a single fabrication mask with accurately spaced gaps. This pattern is first transferred to the top comb layer to form the top comb teeth and a mask for the bottom comb teeth. This mask for the bottom comb teeth is then used to fabricate bottom comb teeth that are aligned automatically to the top comb teeth.